1. Field of Invention
The invention relates to an imaging system which forms an image by exposing microcapsules that encapsulate a photosensitive material, thereby, creating a latent image with the capsules of various mechanical strength, and rupturing the capsules with pressure, so that the image is developed on a print medium. More concretely, the invention relates to the imaging system wherein the method of heating using a heating unit for promoting an image-forming reaction is improved.
2. Description of Related Art
There have been proposed imaging systems based on microcapsules, as disclosed in U.S. Pat. No. 4,440,846 and U.S. Pat. No. 4,399,209.
Such imaging systems form an image by selectively exposing a photosensitive layer, wherein a photosensitive material is encapsulated within microcapsules, to radiation in correspondence with image information, and then, rupturing the capsules with pressure, whereby the photosensitive material flows out so as to produce the image. The mechanical strength of the microcapsules is changed by exposure so that a latent image is formed with the capsules. The capsules that have not been cured or have been plasticized by radiation are ruptured with pressure due to their weak mechanical strength, and release a color-forming material (a color former) therefrom. The color-forming reaction occurs between the color-forming material and a developer, which produces a color image.
A light and pressure sensitive print medium is utilized for the above-described imaging system. The light and pressure sensitive print medium has a layer structure of a sheet-shaped first substrate, a light and pressure sensitive layer which covers the first substrate with the mixture of the microcapsules and the developer, and a sheet-shaped second substrate. This print medium is easy to handle, because a desired image is formed thereon by exposure. The image is formed on the exposed print medium by the color-forming reaction, which gradually occurs between the color-forming material and the developer after a pressure-developing process. Ordinarily, it takes at least about an hour to obtain an acceptable image by this chemical reaction, and takes nearly 24 hours to complete the color-forming reaction so as to form the image with good tonal qualities. For example, the optical density of the image formed on the light and pressure sensitive print medium is initially less than 0.7 in black, and goes up to more than 1.9 after 24 hours, when left behind at room temperature.
Therefore, contrary to the other types of imaging systems that do not make use of the light and pressure sensitive print medium, it is not possible to estimate or use the image immediately after the development, because of its low optical density (which is less than 0.7 as mentioned above). Also, since the initial image is tinged with red, it is necessary to spend a long time to fix the color tone before use and estimation of the image.
In order to solve the above-mentioned problem, there has been proposed a method by which the print medium is heated, for example, at 80 to 90 degrees in order to promote the color-forming reaction. The imaging system using this method has already been manufactured, providing the higher initial density of the image immediately after the development.
However, this method still has a problem concerning the optical density. The optical density of the image is not high enough for practical use after 24 hours, thereby, providing a monotonous pattern. Furthermore, the optical density does not increase more than a fixed value, even if the print medium is heated at a higher temperature than 80 to 90 degrees. The color-forming material is transformed as a result of promoting the color-forming reaction at such a high temperature, and loses the ability to form a desired color. Accordingly, the optical density of the image does not increase enough to form a satisfactory image with this method, although the practical density can be obtained without the heating process as time passes by.